The circuitry of a conventional spectrum analyzer is designed for optimum performance within a limited range of frequencies. For example, the Tektronix 492 Spectrum Analyzer, when used as a stand-alone instrument, is designed to analyze signals having frequency components in the range from 50 KHz to 21 GHz. However, it is frequently desired to analyze signals having components at frequencies higher than 21 GHz. This can be done using a conventional spectrum analyzer in conjunction with a mixer.
A mixer is a heterodyne conversion transducer which receives an input signal at a frequency F.sub.in and a mixing signal at a frequency F.sub.lo and produces an output signal having a series of frequency components at frequencies F.sub.i (i=1 . . . n) where EQU F.sub.i =F.sub.in .-+.iF.sub.lo ( 1)
Thus, the tenth harmonic (i=10) of a mixing signal having a frequency of up to 6 GHz can be used to generate a mixed signal having a frequency of 10 GHz from an input signal component having a frequency of 70 GHz using the conversion EQU F.sub.10 =F.sub.in -10F.sub.lo
The mixed signal is fed to the spectrum analyzer and is analyzed in essentially the same way as an input signal provided directly to the analyzer and having a frequency of 10 GHz, the analyzer being calibrated in terms of the actual frequency of the input signal. Clearly, the conversions represented by subtractions of iF.sub.lo from F.sub.in are only permitted when the result of the subtraction yields a positive value of F.sub.i.
Since the state of the transmission line art is such that signals at frequencies over about 40 GHz cannot be transmitted efficiently using coaxial cable, mixers for extending the frequency range of the Tektronix 492 Spectrum Analyzer are constructed using waveguide. The conventional waveguide mixer comprises a section of waveguide which is flanged for connection to a signal source and is connected to the analyzer by coaxial cable. The active element of the mixer is a probed diode disposed in the waveguide and having one electrode connected to the waveguide wall and its other electrode connected to the core conductor of the coaxial cable.
The probed diode used in conventional waveguide mixers is expensive to manufacture. The probed diode is a chip of semiconductor material of n-type conductivity having at one side a common cathode. A plurality of mutually isolated regions of p-type conductivity are formed on the chip at its other side, and each p-type region is associated with an individual gold anode. The chip is first positioned inside the waveguide section, secured at its cathode to the core of the coaxial cable connector, and is then probed using a metal catswhisker which is electrically connected to the wall of the waveguide to locate an anode associated with a pn junction having acceptable properties. This is a highly skilled and time-consuming operation requiring a delicate sense of touch. Once a satisfactory diode has been found, the catswhisker is clamped to the waveguide wall and the mixer undergoes various tests, several of which are directed towards ensuring that the catswhisker will maintain contact with the selected anode without puncturing the underlying diode. These tests are time consuming, and may reveal quite a high failure rate. Accordingly, the overall cost of using a probed diode is very high.
In addition, the conversion loss (ratio of the power of the signal under test to the power of the mixed signal) of a probed diode waveguide mixer is dependent on frequency to an undesirable extent.
The conversion loss of a waveguide mixer depends to a large extent on the electric field intensity in the region of the diode. The local field intensity can be increased by using a so-called ridged waveguide, i.e., a waveguide having a transverse ridge along a wide wall, and mounting the diode on the wide wall opposite the ridge.
It has been proposed that beam-lead diodes, which are less expensive than probed diodes, be used in waveguide applications, for example, in U.S. Pat. No. 4,246,556 issued Oct. 20, 1981. In the case of that proposal, the diode is packaged inside an annular body of dielectric material. This packaging has proved satisfactory for many purposes, but the annular dielectric body introduces a significant parasitic shunt capacitance which degrades performance at frequencies higher than 40 GHz.
It will be understood by those skilled in the art that electromagnetic radiation having a wavelength (in vacuum) of less than about 1 cm has a frequency of greater than about 30 GHz. A rectangular waveguide has its minimum impedance for a signal having a wavelength equal to twice the width (longer transverse dimension) of the guide, corresponding to a width of 0.5 cm or less for millimeter wave applications. However, a waveguide having a width of 0.5 cm is able to transmit without unreasonable loss a signal having a frequency as low as 18 GHz. The range from 18 GHz to 26 HGz is therefore considered to be the low end of the millimeter wave frequency range, notwithstanding the fact that the corresponding range of wavelengths (in vacuum) is from 1.66 cm to 1.15 cm. Accordingly, in this specification and in the appended claims the term "millimeter wave" is intended to be construed as relating to electromagnetic radiation having a frquency from about 18 GHz to about 220 GHz.